A known type of polyphase machine is the specially designed, permanently energized synchronous machines. In machines controlled by pulse-controlled a.c. converters, a known method described in German Patent 33 45 271 seeks to improve efficiency when the rotational frequency diminishes. In this method, the phases are subdivided into phase sections and the series connection of the individual phase sections are increased until all the phase sections of each phase are connected in series. By doing this, the number of actively dissipative frequency converters can be reduced to one. On the other hand, with higher rotational frequencies (rated speed) and greater torques (rated-load torque), all the phase sections are connected in parallel. Therefore, the determining phase winding number m and the number of slots q remains unchanged.
At a rated speed n.sub.N, a correspondingly high rotary voltage is induced, whereby all m phase windings are switched in parallel through their frequency converters to the supplying direct-current mains. With diminishing rotational speeds n&lt;n.sub.N,the induced voltage declines approximately in the ratio n/n.sub.N, and increasingly the phase windings are switched from parallel operation over to series operation.
As a result of this switching-over of more and more phase windings from parallel to series operation, the number of phase windings finally decreases to m*. The active number of slots therefore climbs to q*, according to the relation m.multidot.q=m*.multidot.q*. By switching over to series operation, using the known method, fewer losses occur as a result of the reduction of the number of frequency converters remaining in operation, and thereby a better efficiency factor is achieved.
Low-loss bistable circuit elements, such as thyristors which can be activated after time delays, are effective to further reduce losses when switching the frequency converters.
There is, however, a need to provide a polyphase machine that improves the efficiency factor to a greater extent than in the prior art, and runs with minimum process noise and small oscillating torques.